This proposed study will investigate the functional organization of the central auditory system in the brainstem of a vocal fish, the plainfin midshipman (Porichthys notatus). Vocalizing species, including humans, are faced with the fundamental problem of segregating concurrent vocal signals during social communication. Recent studies in the midshipman have revealed insights into the neural mechanisms that underlie vocal signal segregation. Specific aims are to: (1) use neuroanatomical methods to determine the central projections to the medulla of physiologically identified afferents from inner ear otolithic endorgans that may serve an acoustic function, (2) characterize the response properties of medullary auditory neurons to individual and concurrent social acoustic signals that mimic natural vocalizations using standard extracellular recording techniques, and (3) use neuroanatomical methods coupled with neurobiotin injections and extra-and intracellular recording techniques to identify the central projections of physiologically-characterized medullary auditory neurons to the midbrain and other brainstem auditory nuclei. This proposed study will determine how the midshipman auditory brainstem processes biologically meaningful stimuli and may provide novel information of how individual and concurrent signals, such as vowels in human speech, are coded and segregated within the vertebrate auditory system.